1. Field of the Invention
This invention relates to optocouplers, in particular to optocouplers incorporating organic electroluminescent devices.
2. Related Technology
Organic light emitting diodes (OLEDs) comprise a particularly advantageous form of light-emitting device. They are easy and cheap to fabricate on a variety of substrates. Organic (which here includes organometallic) LEDs may be fabricated using either polymers or small molecules in a range of colours, depending upon the materials used. Examples of polymer-based OLEDs are described in WO 90/13148, WO 95/06400 and WO 99/48160; examples of so called small molecule based devices are described in U.S. Pat. No. 4,539,507.
At their most basic, organic electroluminescent devices generally comprise an organic light emitting material which is positioned between a hole injecting electrode and an electron injecting electrode. The hole injecting electrode (anode) is typically a transparent tin-doped indium oxide (ITO)-coated glass substrate. The material commonly used for the electron injecting electrode (cathode) is a low work function metal such as calcium or aluminium.
The materials that are commonly used for the organic light emitting layer include conjugated polymers such as poly-phenylene-vinylene (PPV) and derivatives thereof (see, for example, WO-A-90/13148), polyfluorene derivatives (see, for example, A. W. Grice, D. D. C. Bradley, M. T. Bernius, M. Inbasekaran, W. W. Wu, and E. P. Woo, Appl. Phys. Lett. 1998, 73, 629, WO-A-00/55927 and Bernius et al., Adv. Materials, 2000, 12, No. 23, 1737), polynaphthylene derivatives and polyphenanthrenyl derivatives; and small organic molecules such as aluminium quinolinol complexes (Alq3 complexes: see, for example U.S. Pat. No. 4,539,507) and quinacridone, rubrene and styryl dyes (see, for example, JP-A-264692/1988). The organic light emitting layer can comprise mixtures or discrete layers of two or more different emissive organic materials.
Typical device architecture is disclosed in, for example, WO-A-90/13148; U.S. Pat. No. 5,512,654; WO-A-95/06400; R. F. Service, Science 1998, 279, 1135; Wudl et al., Appl. Phys. Lett. 1998, 73, 2561; J. Bharathan, Y. Yang, Appl. Phys. Lett. 1998, 72, 2660; T. R. Hebner, C. C. Wu, D. Marcy, M. L. Lu, J. Sturm, Appl. Phys. Lett. 1998, 72, 519); and WO 99/48160; the contents of which references are incorporated herein by reference thereto.
The injection of holes from the hole injecting layer such as ITO into the organic emissive layer is controlled by the energy difference between the hole injecting layer work function and the highest occupied molecular orbital (HOMO) of the emissive material, and the chemical interaction at the interface between the hole injecting layer and the emissive layer. The deposition of high work function organic materials on the hole injecting layer, such as poly(styrene sulfonate)-doped poly(3,4-ethylene dioxythiophene) (PEDOT/PSS), N,N′-diphenyl-N,N′-(2-naphthyl)-(1,1′-phenyl)-4,4′-diamine (NBP) and N,N′-bis(3-methylphenyl)-1,1′-biphenyl-4,4′-diamine (TPD), provides “hole transport” layers which facilitate the hole injection into the light emitting layer, transport holes stably from the hole injecting electrode and obstruct electrons. These layers are effective in increasing the number of holes introduced into the light emitting layer.
We will now describe a basic structure of a typical OLED. A glass or plastic substrate supports a transparent anode layer comprising, for example, indium tin oxide (ITO) on which is deposited a hole transport layer, an electroluminescent layer, and a cathode. The electroluminescent layer may comprise, for example, a PPV (poly(p-phenylenevinylene)) and the hole transport layer, which helps match the hole energy levels of the anode layer and electroluminescent layer, may comprise a conductive transparent polymer, for example PEDOT:PSS (polystyrene-sulphonate-doped polyethylene-dioxythiophene) from Bayer AG of Germany. The cathode layer typically comprises a low work function metal such as calcium or barium and may include an additional layer immediately adjacent the electroluminescent layer, such as a layer of lithium fluoride, for improved electron energy level matching. Contact wires connected to the anode and the cathode respectively provide a connection to a power source. The same basic structure may also be employed for small molecule devices.
In general, OLEDs consist of a multi-layer sandwich of indium-tin-oxide (ITO) as anode contact, one or more organic layers including a light emitting polymer (LEP) layer, and a metal layer as cathode, deposited on a planar substrate, usually of glass or a high refractive index plastic such as polycarbonate. In so-called “bottom emitter” devices, the multi-layer sandwich is deposited on the front surface of a planar glass substrate, with the reflecting electrode layer, usually the cathode, furthest away from the substrate, whereby light generated internally in the LEP layer is coupled out of the device through the substrate. We will describe an example of a bottom emitter, where light is emitted through transparent anode and substrate and the cathode is reflective. Conversely, in a so-called “top emitter”, the multi-layer sandwich is disposed on the back surface of the substrate, and the light generated internally in the LEP layer is coupled externally through a transparent electrode layer without passing through the substrate. Usually the transparent electrode layer is the cathode, although devices which emit through the anode may also be constructed. The cathode layer can be made substantially transparent by keeping the thickness of cathode layer less than around 50-100 nm, for example.
Organic electroluminescent devices may be advantageously employed in optocouplers, as described, for example, in U.S. Pat. No. 6,509,574. Previously, optocouplers employed inorganic light-emitting diodes. Commercial light emitting diodes (LEDs) typically constitute a p-n junction of inorganic, doped semiconducting materials such as gallium arsenide (GaAs) or aluminium gallium arsenide (AlGaAs). At these junctions between the doped layers, recombination of electrons and holes results in interband emission of light.
Heteroepitaxial growth of direct bandgap III-V compound semiconductors such as GaAs, InP, and GaP on silicon substrates, from which LEDs can be fabricated, yields highly defective material due to mismatches in lattice parameters and thermal expansion coefficients. These LEDs do not perform well, and the silicon circuits are affected during the heteroepitaxy due to the required high growth temperatures (typically >600° C.). Further, achieving good electrical isolation is not easy in these approaches.
As an alternative, III-V LEDs have been integrated with a silicon drive circuit at the package level. But these approaches are expensive and not suitable for wafer-level integration.
Recently, organic light-emitting diodes (OLEDs) have drawn much attention, especially for emissive display applications. Since OLEDs can be fabricated on any smooth surface, such as silicon wafers, and at low (<100° C.) temperatures, they are also very promising for many optoelectronic applications.
FIG. 1 shows an OLED optocoupler according to the prior art. An optocoupler, or optoisolator, is a coupling device in which a light-emitting diode, energized by the input signal, is optically coupled to a photodetector, such as a light-sensitive output diode, transistor or silicon-controlled rectifier. In FIG. 1, the optocoupler, generally designated 100, is constructed on a semiconductor substrate 101. The semiconductor material can be silicon, silicon germanium, gallium arsenide or any other semiconducting material used in photolithographic manufacturing. Integral to substrate 101 is an integrated circuit (not shown in FIG. 1) and a semiconductor device 102 sensitive to electromagnetic radiation in a certain wavelength range. The radiation-sensitive device 102 is frequently referred to as photodetector.
Laying over photodetector 102 is a flat portion of an optically transparent, electrically insulating layer 103, which has first surface 103a and second surface 103b. The radiation-sensitive device 102 is integral with second surface 103b, and the organic diode 104, operable to emit electromagnetic radiation, is integral with the first surface 103a. The radiation-emitting diode is commonly referred to as OLED (organic light-emitting diode). Consequently, photodetector 102 is electrically isolated from OLED 104 and positioned in the path of the emitted radiation.
However, the input current to an optocoupler incorporating an OLED emitting device may not necessarily be the correct value to produce the optimum OLED current.
For a particular device with a standard specified output current (for example 1 mA), the standard specified input current (for example 1 mA) may be too high for the OLED. Driving OLEDs at above their rated currents leads to reductions in device lifetime, at a rate approximately proportional to the square of the driving current. Increasing the device area increases the cost of the device, but reducing the device area (and increasing the brightness of the OLED) reduces the signal to noise ratio, as well as reducing the lifetime of the device. For a given standard input current and output current for an optocoupler, the input current may be too high for the optimum size of OLED.